We usually think of sheep more as followers than leaders, but in a flock one sheep must lead the way. Long ago, it was common practice for shepherds to hang a bell around the neck of one sheep in their flock, thereby designating it the lead sheep. This animal was called the bellwether, a word formed by a combination of the Middle English words belle (meaning "bell") and wether (a noun that refers to a male sheep that has been castrated). It eventually followed that bellwether would come to refer to someone who takes initiative or who actively establishes a trend that is taken up by others. This usage first appeared in English in the 13th century.
In politics, the term is more often applied in the passive sense to describe a geographic region where political tendencies match in microcosm those of a wider area, such that the result of an election in the former region might predict the eventual result in the latter. In a Westminster-style election, for example, a constituency, the control of which tends frequently to change, can mirror in its popular vote the result on a national scale.